<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text" id="txt">
    <button type="button" id="btn">添加数据</button>
    <ul id="demo"></ul>

    <script>
        var txt = document.querySelector('#txt')
        var btn = document.querySelector('#btn')
        var demo = document.querySelector('#demo')
        btn.onclick=function(){
            var con = document.createElement('li')
            con.innerHTML = txt.value;
            document.getElementById('demo').appendChild(con)
        }
    </script>
</body>
</html>